What does 95th %tile mean?

Thu Apr 19 23:54:25 UTC 2001

 > They don't take a one-second sample every five minutes, they take the	
 > five-minute average rate measured by their router.
 >
 > Unless they're insane, or their routers don't support that.  I dunno who
 > makes routers that don't support that, though.

Sorry, perhaps I didn't make the extreme example sufficiently clear:

In the extreme I cited, (full rate for 5 minutes, idle for five minutes, 
repeated),  the five minute average rate oscillates between zero and full 
line rate. The period of oscillation is 10 minutes (i.e. five minutes for 
the five minute rate to decay from line rate to zero and fine minutes to 
build back to line rate).

Now if you sample every five minutes, and the sample point is synchronized 
to the peak and trough of the five minute rate you will get successive 
readings of 'line rate', zero, 'line rate', zero, etc. The 95% sample value 
will be 'line rate'.

If you change _nothing_ except shift the sample point two and one half 
minutes forward in time the sample points will consistently produce 
outcomes of 'half line rate', 'half line rate', ..., and the 95% point is 
'one half of line rate'.

Same algorithm, same raw data, different 95% answers, both valid, yet one 
is twice as large as the other. Great outcome for a billing system isn't it?

(The comment in my earlier note about getting a zero reading requires using 
something other than a 5 minute average data rate. The point I'm trying to 
make in this posting is that even if you do the 'right' thing and collect 
interface data readings every five minutes and do the first order 
differentials yourself to get the five minute data rates, the 95% 'answer' 
is still variable.)